The Gemma M0 (read "M zero") is an update of an earlier Gemma board. Its small size and low price make it perfect for wearables and other projects where you want to build the microcontroller in permanently.
Although tiny, the Gemma has a built-in output in the form of an RGB color-changing LED light. And even though it doesn't have sensors, it does have input -- touch pads that let you trigger events. It also has an on/off switch (something that is always handy).
The big holes around the edges make it easy to connect additional components like lights or servo motors using alligator clip wires, sewable conductive thread, or peel-and-stick foil or fabric conductive tape. It's easy to hold in place on your project with a single glue dot.
How to Power It
For small projects like wearables with lights, you can run Gemma using a case that holds two 3 volt coin batteries (CR 2032).
Suggested Gemma M0 Learning Guides for Beginners
Beginner learning guides for the Gemma M0 tend to be wearables, which means they suggest you cut down on weight by soldering components and using LiPo batteries (which require special recharging).